1. Field of the Invention
The present invention relates to a charge pump circuit for voltage boosting.
2. Description of the Related Art
As a conventional charge pump circuit, there has been known a circuit as shown in FIG. 14 (see, e.g., JP 2718375 B (FIG. 11)).
Specifically, n-channel MOS transistors MN1 to MN5 each having the gate and drain thereof connected to each other are connected in series and capacitor elements C1 to C4 are connected individually to the respective connection points between the gates and drains of the MOS transistors MN2 to MN5. To the CLK and CLKB terminals provided at the other end of the capacitor elements, voltages VCLK and VCLKB having phases opposite to each other are inputted, as shown in FIG. 15.
Each of the gate and drain of the n-channel MOS transistor MN1 is connected to a power source VDD and an output OUT of the boosting circuit is extracted from the source of the MOS transistor MN5. A voltage at a connection point among the source of then-channel MOS transistor MN1 and the gate and drain of the n-channel MOS transistor MN2 is assumed to be V12. When the voltage VCLK is initially in the “L”, i.e., GND state, the voltage V12 is given by the expression (1):V12=VDD−Vtn1   (1).
Next, when the voltage VCLK comes into the “H”, i.e., VDD state, the voltage V12 is given by the expression (2):V12=2*VDD−Vtn1   (2)wherein Vtn1 represents the threshold voltage of the n-channel MOS transistor MN1. A voltage at a connection point among the source of the n-channel MOS transistor MN2 and the gate and drain of the n-channel MOS transistor MN3 is assumed to be V23. When the voltage VCLK is in the “H”, i.e., VDD state, the voltage V23 is given by the expression (3) (the voltage VCLKB at that time is in the “L” state):V23=2*VDD−Vtn1Vtn2   (3)wherein Vtn2 represents the threshold voltage of the n-channel MOS transistor MN2. Next, the voltage VCLK comes into the “L” state, i.e., the voltage VCLKB comes into the VDD state. A voltage at the connection point among the source of the n-channel MOS transistor MN2 and the gate and drain of the n-channel MOS transistor MN3 is assumed to be V23. The voltage V23 is given by the expression (4):V23=3*VDD−Vtn1Vtn2   (4).
Finally, a voltage at the output terminal of the conventional charge pump circuit having a four-stage configuration shown in FIG. 13, i.e., the source voltage V5 of the n-channel MOS transistor MN5 is given by the expression (5):V5=5*VDD−Vtn1Vtn2−Vtn3−Vtn4−Vtn5   (5)wherein Vtn3, Vtn4, and Vtn5 each represent the threshold voltages of the n-channel MOS transistors MN3, MN4, and MN5, respectively.
In FIG. 14, the substrate (well) of each of the n-channel MOS transistors MN1 to MN5 is connected to the ground (GND). In that case, due to the back-gate effect, the values of the respective threshold voltages Vtn1 to Vtn5 of the n-channel MOS transistors MN1 to MN5 increases to be higher than the values of the threshold voltages in the absence of the back-gate effect. Accordingly, the voltage V5 at the output terminal, which is given by the expression (5), decreases.